This invention relates to a process for producing clutch facings, and more particularly to a process for producing a clutch facing which can exhibit good performance without the use therein of any asbestos.
Hitherto, a clutch facing has been produced by dissolving a thermosetting resin such as phenolic resin, a rubber material, a vulcanizing agent, a vulcanization accelerator, a friction improver, a filler and the like in a solvent, causing this solution to adhere to an asbestos substrate, then preforming the structure thus obtained and thereafter molding the preformed structure or preform in a mold under heat and pressure. Recently, however, evidence has been found indicating that asbestos may be harmful (i.e., carcinogenic) to humans. Accordingly, asbestos-free clutch facings have become of interest in recent years.
Asbestos-free clutch facings are produced in each case by bonding a thermosetting resin such as phenolic resin, a rubber material, a vulcanizing agent, a vulcanization accelerator and the like to glass fibers as a substrate, then preforming the resulting structure, and molding the preformed structure or preform in a mold under heat and pressure. Such a process has been proposed in, for example, U.S. Pat. No. 4,130,537. Because the clutch facings comprising glass fibers as a substrate exhibit aggressive behavior during operation, according to the process taught in U.S. Pat. No. 4,130,537, organic fibers such as rayon, hemp and cotton are previously incorporated into glass fibers in an amount of from 10 to 185% by weight based on the weight of the glass fibers, and the resulting composite yarns are dipped in a solvent in which phenolic resin, rubber compositions, other friction improvers and the like have been dissolved or dispersed, to cause these materials to adhere onto the composite yarns. Then a preformed structure is produced and thereafter is subjected to heat and pressure to produce a clutch facing.
According to this process, however, the solvent is used in causing phenolic resins, rubber compositions, other friction improvers and the like to adhere to the composite yarns of the glass fibers and the organic fibers. Moreover, various materials having different specific gravities are contained in the rubber composition. For this reason, the materials having large specific gravities which are contained in the rubber composition sink to the bottom of the solvent, and therefore it is difficult to cause a rubber compound having a uniform composition to adhere onto the composite yarns. Further, it is difficult to control the amount of the materials adhering to the composite yarns.
Thus, there arises a problem in that deviant scattering of the characteristics of the resulting clutch facings tends to occur. Furthermore, since a solvent is used in the adhesion step described above, the solvent may remain in the interior of the clutch facings although the solvent is readily dissipated from the surface during drying and molding. This solvent cannot be completely removed under heat and pressure, and therefore it gradually gasifies during use of the clutch facings by virtue of frictional heat to appear at the surface. Thus, the coefficient of friction is reduced and gives rise to the problem of fading phenomenon.
Japanese Patent Laid-Open Pub. No. 3828/1982 of the present applicant discloses a process for producing a clutch facing which comprises impregnating glass fibers or a composite of glass fibers and asbestos with a solution of thermosetting resins such as phenolic resin, drying the impregnated structure, causing a bulk rubber composition to adhere to the resulting substrate by forming in situ the bulk rubber composition into a sheet by means of rolls, then preforming the resulting structure and thereafter subjecting it to heat and pressure.
However, according to this method, the rubber composition is caused to adhere onto the substrate by forming in situ the bulk rubber composition into a sheet by means of rolls, and therefore it is difficult to obtain a rubber composition having a uniform film thickness. Furthermore, the rubber composition may adhere to the rolls, and the resulting sheetlike rubber composition may be cut on the way. Accordingly, the rubber composition cannot be caused to adhere uniformly onto the glass fiber substrate, and deviant scattering of the characteristics of the resulting clutch facings occurs.
We have carried out further studies in view of these problems and as a result have made the following findings.
(a) U.S. Pat. No. 4,130,537 teaches that organic fibers such as rayon, hemp and cotton must be used in an amount of from 10% to 185% by weight based on the weight of the glass fibers along with the glass fibers because clutch facings comprising a glass fiber substrate exhibit aggresive behavior during operation. However, according to the results of our studies, clutch facings comprising glass fibers do not exhibit aggressive behavior during operation, and in this regard, it is not necessary at all to use organic fibers. When a large amount of an organic fiber is used, the organic fibers are melted or carbonized by frictional heat generated during use of the clutch facing. Thus, the coefficient of friction of the clutch facing is reduced, and the use of a large amount of organic fibers is not desirable.
(b) When the rubber composition preformed into a sheet is used to cause the rubber composition to adhere to the glass fiber substrate, the prior art problems associated with the use of a solvent are solved all at once.
(c) As mentioned above, a uniform rubber composition cannot be caused to adhere to the substrate in the process of causing the bulk rubber composition to adhere to the substrate. This problem is solved by mixing a small amount of organic fibers into the rubber composition and forming it beforehand into a sheet.